JKMakowka wrote: Yeah... details

Should we go on?

So you have your slab that extends 1/3 down the pit, with 5 t of weight on top, and somehow you managed to seal the toilet opening as well. Now you apply the pressure, some of the stuff in the pit will leave the pit via the desired way, the pipe. But some part of the stuff will be pressed in the soil walls of the pit and leave the pit via the easiest way it finds there. What would be the easiest way: deep down towards the underground? Or maybe rather up along the outside wall of the slab?

Yeah... details

A heavy concrete ring and connection to the latrine superstructure plus spiral earth screws might do the trick... ahh well diminishing returns in regards to cost advantage

JKMakowka wrote: Maybe one could make fiber reinforced plastic (FRP) based latrine slabs (nice ones with an included water seal) that extend 1/3 into the pit and can be covered partially by earth or concrete blocks to add weight as an upgrade to existing pit-latines?

Leaving the sealing problems aside, the weight you would need to apply on top would be considerable. For a pressure of 0,5 bar, imagine a column of water of the diameter of your slab, 5 m high.

Hmm, ok. I guess you are right.

Maybe one could make fiber reinforced plastic (FRP) based latrine slabs (nice ones with an included water seal) that extend 1/3 into the pit and can be covered partially by earth or concrete blocks to add weight as an upgrade to existing pit-latines?

To these one could then apply the higher overpressure and maybe they could include the bottom pipe for emptying already (similar to the aqua privi type).
Obviously the costs would be higher, but the resulting latrines would be much nicer to use also...

Ok, it's fun to argue about such things, so let's continue

I think it's quite simple:

You have the latrine on the one side, and a mobile container on the other side. You want to get the stuff from the one side to the other. Now you can do that by applying a vaccum on the one side or pressure on the other. The difference of pressure needed and the volume to be pumped will be the same in both cases. And so will be the needed pressure resistance of your container, be it on the latrine side or on the mobile container side.

You may compensate pressure losses through porosity of soil or unsufficient sealing in your pressure-latrine by pumping higher volumes of air, but then consequently you would need a much bigger pump than for the vaccum tank.

As for the comparison with the biogas tank, I think this doesn't hold: Simple fix-dome biogas digesters need to support pressures of around 0,1 bar max., and to achieve this gas-tight structure is already more complicated and expensive than to construct a latrine. For higher pressures, this will get even more complicated and expensive.

And you need more pressure (as also Marvijn already wrote) to move the stuff in your latrine, due to the viscosity and the density (higher than water!) of the material. The pressure you need is the same as typically needed in vaccum trucks, I guess at minimum 0,5 bar.

Good remarks, you might be right...

@Marijn Zandee:
1. True, but the seepage is rather slow and latrine-pit walls tend to be covered by a pretty tight muck. It would need to be tested, but I guess the short term pressure-loss wouldn't be all too high in most cases.
2. Yeah, that would obviously be a problem, pits would either have to have some water added and stirred(like it is done with vacuum suckers) or maybe emptied more often.
3. Yep but at the top there should be mostly air or steam

@Florian
Well, a transportable steel tank with vacuum-pump is probably a 1000 times more expensive that reinforcing some existing under-ground pits, which are quite pressure resistant by nature, especially if build with a heavy (reinforced concrete) dome-shaped slab. Small concrete biodigesters are actually a case in point, as they often have to sustain similar internal pressures and are not very expensive to build.

But you are right, it might be much more difficult to get the pits somewhat airtight than I am currently thinking.
However due to all the water/moisture and muck in the pit, I am guess that even though it wouldn't be airtight obviously, it would probably not loose too much overpressure in the short run (as that can be applied rather quickly).
Maybe one would have to build them in a way that the upper 1/3 is more or less air and water tight, but that wouldn't effect their slow water percolation function much at all.

Hmm... ahh well... it would need to be tested I guess Too bad that I don't have any pit-latrines close by here in Germany

JKMakowka wrote: Advantages over standard vacuum based pit emptiing would be the much cheaper setup, with no air pump required (on site) and also no vacuum resistant tank for applying the underpressure.

You save 1 vaccuum resistant tank on a truck by installing 1000 pressure resistant tanks in latrines. Not that economic...

The constructive effort to make a latrine air and pressure tight is even higher as for a vaccum tank on the truck, as you need to integrate a big lid (the toilet hole).

And then, as your latrine is air tight, it will also be water tight, which means no percolation of liquids, and therefore much more volumes to evacuate....

Sorry

Dear Julius,

Thanks for putting these brainstorm ideas on the forum, I find them fun to think about.

As an engineer I have a few points that make we wonder about the feasibility of the idea though.

1.) Latrine pits are usually designed such that liquids can seep into the soil, I think it will depend strongly on the type of soil whether you can build up enough pressure in the pit or if there would be too much leakage.

2.) From what I have read the sludge in pits is often close to solid or solid in the lower layers(depending on the infiltration capacity. If you could push those out by pressurizing (which I doubt), it may take quite a bit more pressure than you anticipate as a result of viscosity.

3.) The seal between the pit-lid and the wall needs to be really well made, we don't want a jet of sludge spouting through a small hole.

Regards

Marijn

Another brainstorm topic

So I was reading up on over-pressure based sewerage systems quite popular more than hundred years ago (back when vacuum systems where also invented), and thought this might also be used to empty pit-latrines.

Basically your would only have to make sure your pit can sustain mild overpressures (0.2bars to lift the faecal sludge 2 m to a tank), which should be not a big issue for under-ground pits. With a pressure resistant heavy lid or pit lining designed for this purpose one could apply this overpressure quite simply using an compressed air bottle or typically available compressor for filling car wheels etc.
All else you would need would be a larger diameter pipe going to the bottom of the pit, through which the the faecal sludge is forced out into your mobile container.

One could also use a pressure-cooker with an attached pipe or somthing like that to build up the pressure (water expands 1600 times as steam, so unless my math fails me completely, boiling 2,5l of water would result in 4qm of steam at athmospheric pressure, or 2qm at 2bars etc., which would be more than sufficient to empty a standard sized pit latrine). The steam could be used in addition to sterilize/clean the latrine.

Advantages over standard vacuum based pit emptiing would be the much cheaper setup, with no air pump required (on site) and also no vacuum resistant tank for applying the underpressure.
Disadvantage would be obviously the stress on the pit-lining and that some latrines might not be air-tight enough for it to work. Newly constructed latrines could be build with this in mind however easily I think.

In the long term, such pressure resistant pitlatrines could potentially also be upgraded to become part of a over-pressure sewer system, without much additional cost.

Comments welcome!